Constraining the primordial orbits of the Terrestrial Planets

Abstract

Evidence in the Solar System suggests that the giant planets underwent an epoch of radial migration that was very rapid, with an e-folding timescale shorter than 1~Myr. It is probable that the cause of this migration was that the giant planets experienced an orbital instability that caused them to encounter each other, resulting in radial migration. Several works suggest that this dynamical instability occurred `late', long after all the planets had formed and the solar nebula had dissipated. Assuming that the terrestrial planets had already formed, then their orbits would have been affected by the migration of the giant planets. As a result, how did the orbits of the terrestrial planets change? And can we use this migration to obtain information on the primordial orbits of the terrestrial planets? We directly model a large number of terrestrial planet systems and their response to giant planet migration. We study the change in the Angular Momentum Deficit (AMD) of the terrstrials. We conclude that the primordial AMD should have been lower than ~70\% of the current value, but higher than 10\%. We find that a scenario with five giant planets better satisfies the orbital constraints of the terrestrial planets. We predict that Mars was initially on an eccentric and inclined orbit while the orbits of Mercury, Venus and Earth were more circular and coplanar. The lower primordial dynamical excitement and the peculiar partitioning between planets impose new constraints for terrestrial planet formation simulations.